Apparatus for polishing thin, flat semi-conductor wafers is well-known in the art. Such apparatus normally includes a polishing head which carries a membrane for engaging and forcing a semi-conductor wafer against a wetted polishing surface, such as a polishing pad. Either the pad, or the polishing head is rotated and oscillates the wafer over the polishing surface. The polishing head is forced downwardly onto the polishing surface by a pressurized air system or, similar arrangement. The downward force pressing the polishing head against the polishing surface can be adjusted as desired. The polishing head is typically mounted on an elongated pivoting carrier arm, which can move the pressure head between several operative positions. In one operative position, the carrier arm positions a wafer mounted on the pressure head in contact with the polishing pad. In order to remove the wafer from contact with the polishing surface, the carrier arm is first pivoted upwardly to lift the pressure head and wafer from the polishing surface. The carrier arm is then pivoted laterally to move the pressure head and wafer carried by the pressure head to an auxiliary wafer processing station. The auxiliary processing station may include, for example, a station for cleaning the wafer and/or polishing head; a wafer unload station; or, a wafer load station.
More recently, chemical-mechanical polishing (CMP) apparatus has been employed in combination with a pneumatically actuated polishing head. CMP apparatus is used primarily for polishing the front face or device side of a semi-conductor wafer during the fabrication of semi-conductor devices on the wafer. A wafer is "planarized" or smoothed one or more times during a fabrication process in order for the top surface of the wafer to be as flat as possible. A wafer is polished by being placed on a carrier and pressed face down onto a polishing pad covered with a slurry of colloidal silica or alumina in de-ionized water.
A schematic of a typical CMP apparatus is shown in FIGS. 1A and 1B. The apparatus 10 chemical mechanical polishing consists of a rotating wafer holder 14 that holds the wafer 10 appropriate slurry 24, and a polishing pad 12 which is normally mounted to a rotating table 26 by adhesive means. The polishing pad 12 is applied to the wafer surface 22 at a specific pressure. The chemical mechanical polishing method can be used to provide a planar surface on dielectric layers, on deep and shallow trenches that are filled with polysilicon or oxide, and on various metal films. CMP polishing results from a combination of chemical and mechanical effects. A possible mechanism for the CMP process involves the formation of a chemically altered layer at the surface of the material being polished. The layer is mechanically removed from the underlying bulk material. An altered layer is then regrown on the surface while the process is repeated again. For instance, in metal polishing a metal oxide may be formed and removed repeatedly.
A polishing pad is typically constructed in two layers overlying a platen with the resilient layer as the outer layer of the pad. The layers are typically made of polyurethane and may include a filler for controlling the dimensional stability of the layers. The polishing pad is usually several times the diameter of a wafer and the wafer is kept off-center on the pad to prevent polishing a non-planar surface onto the wafer. The wafer is also rotated to prevent polishing a taper into the wafer. Although the axis of rotation of the wafer and the axis of rotation of the pad are not collinear, the axes must be parallel. Polishing heads of the type described above used in the CMP process are shown in U.S. Pat. Nos. 4,141180 to Gil, Jr., et al.; 5,205,082 to Shendon et al; and, 5,643,061 to Jackson, et al. It is known in the art that uniformity in wafer polishing is a function of pressure, velocity and the concentration of chemicals. Edge exclusion is caused, in part, by non-uniform pressure on a wafer. The problem is reduced somewhat through the use of a retaining ring which engages the polishing pad, as shown in the Shendon et al patent.
Referring now to FIG. 1C, wherein an improved CMP head, sometimes referred to as a Titan .RTM. head which differs from conventional CMP heads in two major respects is shown. First, the Titan .RTM. head employs a compliant wafer carrier and second, it utilizes a mechanical linkage (not shown) to constrain tilting of the head, thereby maintaining planarity relative to a polishing pad 12, which in turn allows the head to achieve more uniform flatness of the wafer during polishing. The wafer 10 has one entire face thereof engaged by a flexible membrane 16, which biases the opposite face of the wafer 10 into face-to-face engagement with the polishing pad 12. The polishing head and/or pad 12 are moved relative to each other, in a motion to effect polishing of the wafer 10. The polishing head includes an outer retaining ring 14 surrounding the membrane 16, which also engages the polishing pad 12 and functions to hold the head in a steady, desired position during the polishing process. As shown in FIG. 1C, both the retaining ring 14 and the membrane 16 are urged downwardly toward the polishing pad 12 by a linear force indicated by the numeral 18 which is effected through a pneumatic system.
In the improved CMP head 20 shown in FIG. 1C, large variations in the removal rate, or polishing rate, across the whole wafer area are frequently observed. A thickness variation across the wafer is therefore produced as a mean cause for wafer non-uniformity. The improved CMP head design, even though utilizing a pneumatic system to force a wafer surface onto a polishing pad, the pneumatic system cannot selectively apply different pressure at different locations on the surface of the wafer. For instance, as shown in FIG. 4, a profilometer data obtained on an 8-inch wafer is shown. The thickness difference between the highest point on the wafer and the lowest point on the wafer is almost 2,000 .ANG. yielding a standard deviation of 472 .ANG., or 6.26%. The curve shown in FIG. 4 is plotted with the removal rates in the vertical axis and the distance from the center of the wafer in the horizontal axis. It is seen that the removal rates at the edges of the wafer are substantially higher than the removal rate at or near the center of the wafer. The thickness uniformity on the resulting wafer after the CMP process is therefore very poor.
In the conventional polishing head of FIG. 1C, a wafer is held in the polishing head by a carrier film sandwiched between the wafer and the chuck. The carrier film provides the necessary elasticity. The rigid chuck and the carrier film apply uniform stress on the back of the wafer. However, it is frequently not possible to guarantee uniform polishing over the entire wafer surface due to other processing variables. For instance, the polishing slurry may not have been uniformly distributed under the entire wafer area, the pre-polishing thickness profile of the wafer prior to the CMP process may not have been uniform, and furthermore, the pad conditioning profile also may not have been uniform.
It is therefore an object of the present invention to provide an apparatus for controlling polishing profile on a substrate in a polishing process that does not have the drawbacks or shortcomings of the conventional apparatus.
It is another object of the present invention to provide an apparatus for controlling the polishing profile on a wafer during a CMP process that is capable of applying a selectively non-uniform pressure on the wafer surface.
It is a further object of the present invention to provide an apparatus for controlling a polishing profile on a wafer during a CMP process which is capable of applying a different pressure at the center of the wafer than that on the remaining area of the wafer.
It is another further object of the present invention to provide an apparatus for controlling a polishing profile on a wafer during a CMP process which utilizes an elastic plate as a backing plate for the wafer and a contour adjusting means for applying a pressure on the elastic plate to effectuate control of the contour of the wafer.
It is still another object of the present invention to provide an apparatus for controlling a polishing rate on a wafer during a CNP process which includes an elastic plate having sufficient rigidity made of BeCu alloy for changing the contour of the wafer.
It is yet another object of the present invention to provide an apparatus for controlling a polishing profile on a substrate during a CMP process which incorporates the use of a carrier film positioned between a wafer and an elastic plate for absorbing impact on the wafer during the polishing process.
It is still another further object of the present invention to provide a method for controlling a polishing profile on a substrate during a polishing process by mounting an elastic plate of sufficient rigidity behind a substrate and then deforming the elastic plate such that the contour of the substrate may be changed accordingly for changing the polishing profile.
It is yet another further object of the present invention to provide a method for controlling a polishing profile on a silicon wafer during a CMP process by utilizing an elastic plate and a carrier film behind the wafer such that the shape of the elastic plate may be changed from being concave to being convex to subsequently changing the polishing profile on the wafer surface.